The study of Seyfert 2 galaxies with and without infrared broad lines

Abstract

From the literature, we construct a sample of 25 Seyfert 2 galaxies (S2s) with a broad line region (BLR) detected in near infrared (NIR) spectroscopy and 29 with NIR BLRs which were not detected. We find no significant difference between the nuclei luminosity (extinction-corrected [OIII] 5007) and infrared color f60/f25 between the two populations, suggesting that the non-detections of NIR BLRs could not be due to low AGN luminosity or contamination from the host galaxy. As expected, we find significantly lower X-ray obscurations in Seyfert 2s with NIR BLR detection, supporting the unification scheme. However, such a scheme was challenged by the detection of NIR BLRs in heavily X-ray obscured sources, especially in six of them with Compton-thick X-ray obscuration. The discrepancy could be solved by the clumpy torus model and we propose a toy model demonstrating that IR-thin X-ray-thick S2s could be viewed at intermediate inclinations and compared with those IR-thick X-ray-thick S2s. We note that two of the IR-thin X-ray-thick S2s (NGC 1386 and NGC 7674) experienced X-ray transitions, i.e. from Compton-thin to Compton-thick appearance or vice versa based on previous X-ray observations, suggesting that X-ray transitions could be common in this special class of objects.